Dllmain function is not being called

Question

I searched here but none of these questions helped me so yeah I'll explain: My Dllmain function is not being called when it attaches to a process (rundll32.exe) in visual studio project settings I changed it to attach to rundll32.exe it was supposed to show a messagebox on attach but it just doesn't do it. My code:

// dllmain.cpp : Defines the entry point for the DLL application.
#include "pch.h"

BOOL APIENTRY DllMain( HMODULE hModule,
                      DWORD  ul_reason_for_call,
                      LPVOID lpReserved
                    )
{
   switch (ul_reason_for_call)
   {
   case DLL_PROCESS_ATTACH:
       MessageBox(NULL,L"ThumbsUp",L"Attached",MB_ICONINFORMATION);
   case DLL_THREAD_ATTACH:
   case DLL_THREAD_DETACH:
   case DLL_PROCESS_DETACH:
       break;
   }
   return TRUE;
}

Thanks

Do not use `MessageBox` to debug issues concerning `DllMain`. Use [OutputDebugString](https://stackoverflow.com/questions/3179199/simplest-way-to-write-output-message-to-output-window-in-visual-studio-2010). I bet the function *is* being called. — PaulMcKenzie, Aug 17 '21 at 09:28
@PaulMcKenzie thanks but it didn't work my output is like this https://pastebin.com/raw/S7QYXpe5 And my code is like this: https://pastebin.com/raw/HXVWEqSm I don't see any "I'm running" in output — SadLandscape, Aug 17 '21 at 09:33
If you use rundll32, make sure you run it like this `rundll32 blahblah.dll,something` otherwise, rundll32 will do nothing. — Simon Mourier, Aug 17 '21 at 09:45
in order to some function from dll will be called, need that dll itself first be loaded to process. your dll is loaded ? based on https://pastebin.com/raw/S7QYXpe5 - no. so what you want ? — RbMm, Aug 17 '21 at 10:09
*"My Dllmain function is not being called when it attaches to a process"* - What are you doing that causes a process to actually load your DLL? Vital details like this are missing from the question, so it's hard to even guess what's wrong (the code, or your expectations). — IInspectable, Aug 17 '21 at 10:58
@IInspectable I used visual studio's attach to process debugging settings — SadLandscape, Aug 17 '21 at 11:41
@RbMm I used visual studio's attach to process debugging settings — SadLandscape, Aug 17 '21 at 11:41
@SimonMourier is there any way to do that in visual studio so I don't have to build everytime then run it? — SadLandscape, Aug 17 '21 at 11:42
*I used visual studio's attach to process debugging settings* ?!? — RbMm, Aug 17 '21 at 11:50
@RbMm oh I tought I inject my dll to rundll32.exe so all this time I was debugging rundll32.exe? — SadLandscape, Aug 17 '21 at 12:12
@RbMm Thanks, can you please tell me how can I debug dll instead? I wanna test it to see if it works or not Thanks — SadLandscape, Aug 17 '21 at 12:17
Sure, just do this like this: https://i.imgur.com/A25CkFx.png this should trigger the dll load — Simon Mourier, Aug 17 '21 at 12:33
@SimonMourier Thank you so much post this as an answer and I'll mark it as an answer — SadLandscape, Aug 17 '21 at 12:38
@SimonMourier after running it displays messagebox but after clicking ok it gives me error saying invalid entry x should I replace the "x"? — SadLandscape, Aug 17 '21 at 12:43
@SimonMourier after running it displays messagebox but after clicking ok it gives me error saying missing entry x should I replace the "x"? — SadLandscape, Aug 17 '21 at 12:43

score 1 · Accepted Answer · answered Aug 17 '21 at 12:51

If you use rundll32.exe as the loader program, you must call it using an entry point, like this:

In this case, DllMain has been called, but you'll get a messagebox error like :

Error in D:\blah\blahblah.dll
Missing entry: x

That's normal, it's because you don't have exported any externally callable function from your .dll. "x" is here just a placeholder to force rundll32 to load the .dll. If you really want to use rundll32, then read this: How does RunDll32 work?

Dllmain function is not being called

1 Answers1